Staphylococcus aureus is the most common cause of skin and soft tissue infections (SSTIs) in humans, which result in 14 million outpatient visits and half of a million hospital admissions per year in the U.S. Moreover, the epidemic of community-acquired methicillin-resistant S. aureus (CA-MRSA) in the past 2 decades has become an enormous public health threat, as virulent and antibiotic resistant strains are causing severe and necrotic SSTIs in healthy people outside hospital settings (and without risk factors for infection). The experiments in this proposal will build on the observations in humans with the rare orphan disease Hyper-IgE syndrome, who suffer from an eczema-like skin eruption and chronic and recurrent skin infections with S. aureus. These patients were found to have loss-of-function mutations in the signaling molecule STAT3 and a deficiency in Th17 cells, suggesting that STAT3 and IL-17/Th17 responses are essential for cutaneous host defense against S. aureus. Although IL-17/Th17 responses in skin have been implicated in the chronic inflammatory skin disease psoriasis, little is known about the host defense role of IL-17/Th17 responses during an acute MRSA skin infection. The overall hypothesis of this proposal is that STAT3 and IL-17/Th17 protective cutaneous immune responses against MRSA in skin are induced by cytokine cross-talk between keratinocytes and T cells. This study has three aims. Aim 1 will evaluate the host defense role of STAT3 in keratinocytes. Aim 2 will evaluate the differential role of STAT3 in T cell subsets during primary infection and reinfection of MRSA in skin. Aim 3 will determine the mechanisms of STAT3 in human keratinocytes to promote host defense mechanisms, including antimicrobial peptide production and neutrophil responses. To test these hypotheses, we have developed an innovative mouse model S. aureus/MRSA skin infection that combines light-emitting bacteria with in vivo optical imaging to noninvasively monitor the bacterial burden in the infected skin by measuring the amount of emitted light in anesthetized mice over time. In addition, a human skin/SCID mouse xenograft model, human organotypic keratinocyte cultures and keratinocytes from Hyper-IgE syndrome patients. This study will provide a broad and sustained impact to the field by defining the key cutaneous host defense mechanisms against S. aureus/MRSA, an important human bacterial skin pathogen. The proposed experiments will define the cytokine interactions induced by STAT3 between keratinocytes and T cells to promote host defense against MRSA in the skin. Finally, this study may uncover novel cutaneous immune mechanisms that could be targeted in the future development of immune-based therapeutic strategies to help combat S. aureus/MRSA infections in humans.